BM separates like all milks do with the creamy bit at the top. We just aren't used to seeing it with cows milk anymore (unless you get the unhomogonised organic stuff like I do ).
Gently swirling it to mix is fine. Don't shake it though - ruins some of it's properties:
www.bflrc.com/ljs/breastfeeding/shakenot.htm
The 48 hours limit on fridged milk is only really for hospitalised, prem or sick babies as they are more at risk or infection. DOH guidelines state 48 hours to allow for this worst case scenario...If you baby is healthy & at home then you can store milk for up to 8 days in a fridge at 0-4 degrees (in the main compartment not the door) so yours should be fine if it was expressed only yesterday.
The best way to know if your milk has spoiled is to taste/smell some when it's fresh...and label any expressed milk so you know how old it is.
Kellymom has an excellent section on expressing, storing & re-using bm: www.kellymom.com/bf/pumping/index.html
These pages from it are particularly relevant to this discussion:
www.kellymom.com/bf/pumping/milkstorage.html
www.kellymom.com/bf/pumping/reusing-expressedmilk.html